Regenerative metal-air electrochemical cells operate through the oxidation and reduction of a metal fuel. In some metal-air cells, such as that disclosed in U.S. Provisional Patent Application No. 61/054,364, filed on May 19, 2008, and U.S. patent application Ser. No. 12/385,489, filed on Apr. 9, 2009, the contents of both of which are incorporated herein by reference in their entireties, the initial discharge of the electrochemical cell occurs through the oxidation of a metal to a metal oxide on an electrode acting as a fuel anode, while an air cathode reduces an oxidizer, such as oxygen to hydroxide, in the flowing electrolyte solution. As the metal fuel is increasingly oxidized, the capacity of the cell to discharge energy to an external load is progressively reduced. By utilizing a third electrode as an oxygen evolution anode, and utilizing the depleted fuel anode as a plating cathode, the application of a power source can recharge the cell by reducing an oxidized form of the metal to metal fuel, and evolving oxygen.
Oxygen evolved during recharge of such electrochemical cells typically gasses out of the electrolyte and is discarded to the atmosphere. In such electrochemical cells, the virtually limitless supply of oxygen in the air ensures that the air cathode will constantly have a source of oxygen needed for the reduction of oxygen to hydroxide during discharge. One potential drawback of using air as a source of oxygen is that generally less than 21 percent of dry ambient air is composed of the oxygen used by the electrochemical cell. The other gasses present in air do not further the electrochemical reactions in the cell, and may detract from the operation of the cell over time. For example, carbon dioxide, which constitutes approximately 0.04 percent of dry air, may react adversely in the electrochemical cell to create carbonate on the air cathode. The gradual buildup of carbonate may increasingly degrade the efficiency of the electrochemical cell, thereby limiting the cell's effective lifespan. While continuously supplying pure oxygen from an external source to the air cathode during its operation would eliminate the adverse effects stemming from the presence of gasses other than oxygen in ambient air, such a supply may increase both the size and cost of the cell, making such cells inappropriate for a multitude of uses.
The present invention endeavors to provide various improvements over regenerative electrochemical cells utilizing an air cathode.